In general, a conventional switching power supply having two or more out voltage lines on the secondary side of one transformer has a configuration in which feedback control is performed for a primary side control circuit in order to obtain stable constant voltage at the side at which load power is high, and other output voltages are made to pass through a step-down circuit such as a dropper circuit or chopper circuit for obtaining stable voltages, respectively. However, in the above configuration, it is inevitable that the dropper circuit constantly generates power loss or the chopper circuit generates loss at conversion time.
FIG. 17 illustrates an example of 12 V output current vs. 12 V output voltage characteristics obtained in the case where the secondary side has two outputs of 12 V and 24 V and where 24 V output is set as a stabilized output (hereinafter, output that is fed back to the primary side control circuit is referred to as “stabilized output”) and 12 V output is set as a non-stabilized output (the other output that is not fed back to the primary side control circuit is referred to as “non-stabilized output”). FIG. 18 illustrates an example of 24 V output current vs. 12 V output voltage characteristics obtained in the case where the secondary side has two outputs of 12 V and 24 V and where 24 V output is set as the stabilized output and 12 V output is set as the non-stabilized output. Thus, the voltage of 12 V output which is the non-stabilized output varies depending on the load state of 12 V output or 24 V output.
There exists an efficiency-oriented method that improves the winding voltage and structure of a transformer to reduce a variation in a secondary side winding so as to suppress a voltage variation of the non-stabilized output with respect to a current change of the stabilized output. However, the voltage accuracy of the non-stabilized output is worse than that of the stabilized output and, thus, the variation range of output voltage becomes wide. Further, when the current of the stabilized output reaches an overcurrent region, surge voltage occurs in a transformer winding of the non-stabilized output due to influence of leakage inductance existing in the transformer, significantly increasing the voltage of the non-stabilized output. Further, in the case where the non-stabilized output is stabilized by the dropper circuit, the input voltage of the dropper circuit sometimes rises to exceed a maximum rated voltage.
In order to solve the above problems, conventional arts such as Patent Documents 1 to 3 propose a method in which power is supplied from a relatively high voltage side output to a relatively low voltage side output through a dropper circuit so as to improve the voltage accuracy of the non-stabilized output.